page_title: HVDC Inter-Island
section_title: HVDC transmission line
hierarchical_section_title: HVDC Inter-Island / Technical description / HVDC transmission line
caption_attribution_description: English: Sample cross-section of a typical ACSR conductor for a 500 kV DC overhead power line (from Benmore–Haywards link/HVDC Inter-Island transmission line, New Zealand). The conductor consists of 7 strands of steel (centre, high tensile strength), surrounded by 4 outer layers of aluminium (high conductivity). Sample diameter 40 mm. The red is just tape used to fix the sample and doesn't actually occur on the product.
context_page_description: The HVDC Inter-Island link is a 610 km long, 1200 MW bipolar high-voltage direct current transmission system connecting the electricity networks of the North Island and South Island of New Zealand together. It is commonly referred to as the Cook Strait cable in the media and in press releases, although the link is much longer than its Cook Strait section, and the submarine section consists of 3 parallel cables. The link is owned and operated by state-owned transmission company Transpower New Zealand.
  The HVDC link starts at the Benmore Hydroelectric Power Station, on the Waitaki River in Canterbury, on the South Island, and then it travels 534 kilometres on an overhead transmission line through inland Canterbury and Marlborough to Fighting Bay in the Marlborough Sounds. From Fighting Bay, the link travels 40 km via submarine cables underneath Cook Strait to Oteranga Bay, near Wellington, before travelling the final 37 km on overhead lines to Haywards transmission substation in Lower Hutt.
  The HVDC link first became operational in April 1965 to primarily transport electricity from the generation-rich South Island to the more populous North Island.
context_section_description: The transmission line was designed and built by the New Zealand Electricity Department, and was completed in January 1965. The original construction of the line included the erection of 1623 steel lattice towers. In some South Island sections, the line reaches an altitude of 1280 metres. The longest span is 1119 m, near Port Underwood, close to the Fighting Bay cable terminal station.
  The line was originally designed for operation at ±250 kV. During the DC Hybrid link project of 1989 to 1992, the transmission line was reinsulated with DC fog type porcelain insulator units, to enable operation at 350 kV. There are 15 units per insulator string in the inland parts of the route, and 33 units per insulator string in the coastal parts of the route that are exposed to salt condensation. The insulator strings in the coastal portions are around 5 m long.
  The transmission line insulators support a pair of ACSR conductors on each side of the towers. The conductors are each 39.4 mm in diameter, and are spaced 432 mm apart.
  The HVDC line has a continuous overhead earthwire for lightning protection, except for a 21 km section at the Haywards end, where the line is shielded by the electrode line conductors. A 13 km section of the North Island HVDC line uses an overhead earthwire containing a fibre optic core (OPGW), and a further 169 km section of OPGW is installed the South Island line.
  About 20 new towers were constructed in 1992 to reroute the HVDC line north of Johnsonville to make way for new residential development. This was known as the Churton Park deviation.
  Around 92.5 percent (1503) of the towers on the line in 2010 were identified as being original, with the remaining towers having been replaced due to line deviations, collapse, or corrosion.
  Following the DC Hybrid Link project, the line was rated to carry 2000 amperes continuously on each pole, at an operating voltage of 350 kV HVDC.
caption: Sample cross-section of HVDC line conductor
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